The line passes through point (0,2) and (4,1)
The slope of the line = Change in y ÷ Change in x
Slope(m) = (2 - 1) ÷ (0 - 4) = 1/-4 =

Product of two perpendicular lines should be -1
Let the slope of the perpendicular line, to our line, be x
So x × -1/4 = -1
x = -1 ÷ -1/4 = 4
The slope (x) of the perpendicular line is 4